Acoustic well logging



Feb. 19, 1952 F. N. TuLLos ACOUSTIC WELL LOGGING Filed NOV. 26, 1947 5.5152 l man :25; W

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IIIILIIIII IIIIIIJ AGENT.

Patented Feb. 19, 1952 2,586,745 ACOUSTIC WELL LoGGING Frank N. Tullos, Houston, Tex., assignor, by

mesne assignments, to Standard Oil Development Company, Elizabeth, N. J., a corporation of Delaware Application November 26, 1947, Serial No. 788,259

7 Claims. (Cl. 178-44) J The present invention is directed to improvements in self-controlled oscillators. More particularly, the invention is directed to improvements in a magnetostriction transducer involving a selfcontrolled magnetostriction oscillator. In a particular embodiment, the invention relates to imferred to as self-controlled oscillators in which a small portion of the output energy is fed from the output circuit of the network into the input circuit thereof. It is also well known to control the frequency of the energy by electro-mechanithe output circuit through a magnetorestrictive element, which is resonant; at the desired frequency, and feeding this electro-mechanically coupled energy into the input circuit of the network.

In copending application Serial No. 597,408, filed June 4, 1945, for Clare H. Kean, and now abandoned, there is described a 'method for logging subsurface formations penetrated by a borehole by moving through the borehole a magneto- Strictive element capable of generating cylindrically symmetric acoustic waves and measuring variations in electrical characteristics of the magnetostrictive device, which variations are a function of acoustic waves reflected back from the subsurface formations into the magnetostriction transducer.

yIncopending application Serial No. 763,940, liledJuly 26, 1947, for said Kean, now PatentNo. 21,530,971, November 2l, 1950, various embodiments of apparatus suitable for the practice of the aforementioned method are disclosedand claimed. One of the disclosed embodiments includes a self-controlled electron discharge tube oscillator having a pair of solenoids electromechanically coupled to each other through an elongated magnetostrictive element. The magnetostrictive element and oscillator are enclosed measuring vequipment arranged at the surface of "the earth. The present invention is concerned vwith novel improvements which are particularly cally coupling a portion of the energy taken from I 2 useful in apparatus and in the practice of certain aspects of the method disclosed in the aforementioned applications.

It is one object of the present invention to provide means for controlling a self-excited electron discharge tube oscillator whereby the voltage output of said oscillator remains substantially constant with variations in output loading.

It is another object to provide meansrior controlling magnetostriction transducers, Yhaving electron discharge tube driving means-,whereby the excitation and hence the power output of said transducer remains substantially constant with variations in load.

It is still another object of my invention to provide improvements in a well logging device employing a magnetostriction transducer whereby the power output thereof is maintained substantially constant and the sensitivity of measurements made therewith is improved.

It is a further object of my invention to provide improved apparatus for producing a continuous record, or log with respect to depth in a borehole, of a value which is proportional to a functionV of the acoustic impedance of successive earth formations along the borehole.

Other and further objects of my invention will bel apparent from the following description when read in conjunction with the accompanying drawing in which Fig. 1 is a schematic wiring diagram of a selfexcited magnetostriction oscillator or transducer and associated elements including the improvements of my invention, and

Fig. 2 is an elevation view, partly in section, of a well logging device adapted to be used with the apparatus shown in Fig. 1 when logging subsurface formations penetrated by a borehole.

Referring first to Fig. 1, the numeral I0 designates a magnetostrictive metal element such as a nickel, chromium, orv chromium-nickel alloy tube or bar. Element IIJ 'is polarized by magnetic means Il represented schematically as a permanent magnet adjacent element I0. Electro-magnetically coupled to element Il] is a rst solenoid I2 which is electrically connected as an impedance in the output anode or plate circuit of an electron discharge tube I3 by means of conductor I4. Solenoid I2 is also connected by means of conductor I5 to a source of direct current as will be more fully explained hereinafter.

Tube I3 may be any suitable type of electron discharge tube such as a conventional triode. tetrode or pentode, capable of developing sulcient alternating current voltage output so that,

nal or control grid 23 which may be connected' Y through conductor 24 to a coupling capacitor 25 and through resistor 26 vto common ground. Cathode bias resistor 22 is preferably by-passedv by a capacitor 21. It will be seen that solenoid I2, conductors I4 and I5, anode I6, and capacitor I1 constitute the anode output circuit of the electron tube network associated with tube I3 andA that the circuit is completed through a power source andthence through resistor 22 and cathode 2|. Likewise signal grid 23, conductor 24 and resistor 26 constitute the signal input circuit of." then network. Asv will be apparent to workers in the art, solenoid I2 and element I0. constitute means for deriving, output energy from the network associated with tube I3.

If desired,` electron tube I3 may bereplacedby a, pluralityA ofY electron discharge tubes` having theirrespective elements connected in parallel in amanner wellknown in the art when it is Vdesired oiobtainlgreater power output thanis available from afgsingle tube.

Electromagnetically coupled to lelement I9 is arsecond4 solencid-28Ihaving one end connected to common ground andY the other end4 connected by means of conductor 29 to thesignal grid 30, of an electron discharge tube 3| which ispreferably of the triode type-having anganode 32 and agcathode 33. In Fig. 1,'a signal,v grid 3|)V is shown as. providedA with a by-pass capacitorl 34, connected.A to cathode33 and common ground. Also, anode 32 isshownconnected through conductor 35 and load resistor;36,v to conductor |leading toa. source of directv current power. Thus, v solenoid 28, conductor 29, signal grid 30, cathode 33-,and capac tor 34v constitute the signal-,inputcircuitof the networkassociated with tube; 3|, while anode 3 2., conductor 3 5,l resistor 3.6, conductor, I5 and cathode 33 constitute the anode output circuit which isfcoinpletedthrough a power source.

Itgwill-be understood by skilledworkers in the a .r t;` thatthe number of elements Within tube 3| and their connection to othervelements may be varied4 soflongfasftubel is arranged.. to produce across loadvr'esistor 36v an amplifiedl reproduction o fthe alternating current voltagel derivedv from solenoid 28V.

In, the prior art it; is conventional to connect :anode32 and load resistor 33 through conductor 35. to coupling*Y capacitor 25 whereby tl' 1e.,.alter., hating component of voltage developed across resistor 36 is impressed upon the signalv grid of tube I3. The solenoid-28,lby virtue of its electromechanical coupling to solenoid I2 through element, III, constitutes means for coupling aportionr of the output energy derived from tube I3 into theinput circuit ofV tube 3| to sustain oscillation in thesystem. It willbe seenthat, when a large amount,of4 energy. is absorbed, from element I0, the energy electromechanically coupled back from solenoid I2 to solenoid 28. through element Il)y isv reduced and,laccor,dingly, the voltage genf erated, in ysolenoid 28` by, virtue ofl the movement of, slightly magnetized.- element IEI within the 'electromagnetic eld of solenoid 28 is also re- 4 duced as compared with conditions when little energy is absorbed from element l0; The reduced voltage across solenoid 28 results in a reduced alternating voltage output across anode load resistor 36 and thus a reduced voltage across the signal grid circuit o f tube I3. This latter reduced voltage results-in a reducedf voltage output from tube I3 across solenoid I2. Under certain conditions, sucient energy may be absorbed from element I0 so that the system, including the networks yassociated with tubes I3 and 3| will ceaseto produceoscillations, or the useful energy output of the system Vwill fluctuate widely- Inn accordance with my invention I insert between the; output anode circuit of tube 3| and the signal grid or input circuit of tube I3 a variable gain amplifier whose voltage gain is inversely proportional to its input voltage. As will be seen from Fig. 1, I connect the junction between anode 32, andA load resistor 36 to. aV coupling capacitor 3,1 which is electrically connected' by means of conductor 38 to signal' grid 39 of' a third' electron discharge. tube 49. Tube 4I'l is preferably a variable-mu pentode or tetrode of conventional conf struction. In a preferred embodiment of' my invention, the cathodey 4| and4 the suppressor grid 42 of tube 4'0 are joined together and connected t o common ground by conductor 43; The anode 4 4 is connected by means ofconductor 45 to one end of a plate load resistor 46. and also to coupling vcapacitor 25, The other end of load resistor 43 is connected' toconductor I5 and thence to the positive terminal ofA the direct current` power source. Similarlyscreen grid 41. is by-passed to common ground by capacitor 48and is connected to conductor I5 by conductor 49 having in series therewith voltage droppingresistor 50.

' The junctionbetwe'en'A capacitor 3'I and signal grid 39 is connected to common. ground through one or more resistors 5|a. and 5Ib connected in series.V As will be more fully explained hereinafter, measurements orvariations involtage gerrferated in` solenoid 28 and amplified inv the net;- work associatedwithV electron discharge tube 3| may be obtained by a vacuum tube voltmeter connectedacrossresistors 5 I a andSIb or more preferably across resistor 5|b. The provisionof con.- trol tube 4I! and its associated network between the outputircuit ottube 3| andv the input circuit of tube I3 causes the outputenergy derivedfrom tube I3 to be maintained substantially constant rirrespective of variationsin the amountof energy absorbed from element I0. j In a preferred embodiment of my invention arranged for logging subsurface formations pena trated by a borehole, the elements described in the previous paragraphs, together with a., direct current power sourceand a special lter network `(designated collectively. within the broken line 'I0 oflig, 1) are arranged within a.4 cylindrical .housing to be described in conjunction withFig.

having an accurately controlled frequency of about 400 cycles although other frequencies may .be selected. The secondary, windingl of. trans,-

'former 51 is connected through conductor 58 to a condenser 59 which is connected'to conductor 53 of cable 52. The alternating current power from source 55 is thus conducted along cable 52 and is preferably passed through a series-resonant network comprising capacitor 69 and iron core inductor 6| tuned to pass alternating currents havinga frequency equal to that of the cur.- rent generated in power source 55. The output of this network may then be rectified in a con.- ventional voltage doubler rectifier and filter system, designated generally by the elements enclosed within the broken line rectangle 62 in Fig. 1. to supply direct current power to conductor I5.

In order to measure variations in energy which may be reflected from subsurface formations and cause damping of the vibrations of element I and thereby cause changes in the voltage generated in solenoid 28, the varying potential developed across resistor b1., ispassedthrough .conductor 63 into Ia conventional attenuator-lter network comprising elements such asy are schematically represented within the broken lines designated by the numeral 64. Filter network 64 is arranged to attenuate voltages having -a frequency equal to that of power source 55 but to pass other voltages. The direct current voltage .entering lter network 64 through conductor 63 is fed into conductor 53 of cable 52 and may be measured by a conventional electron discharge tube voltmeter including elements such as are shown enclosed within the broken line 65.. To prevent alternating current voltages havingv a frequency equal to thatof power source 55 from entering voltmeter 65, a filter network 66, substantially identical to lter network 64, is interposed in conductor 61 which connects measuring vmeans 65 to conductor 53 of cable 52.

It will be apparent that measuring means 65 is preferably provided with a continuous recording galvanometer 68 capable of producing a permanent record of changes in current plotted against position o f the logging device in a manner well known in the well logging art. For simplification in the drawing, means for correlating the position of the well logging device with the measurement have not been shown.

Although in the schematic representation of measuring means 65power for the operation of the electron discharge tube voltmeteris shown as being derived from batteries it will be .understood that this power maybe suitably drawn Lthrough rectifiers and filters from source 55. Likewise, although conventional heaters and means for heating them are not shown in conjunction with tubes I 3, 3| and 46, it will be understood that these tubes are provided with electric heaters or filaments which may be energized by current from the output of inductor 6| through a conductor 69.

In the foregoing paragraphs I have described,

the network associated with tube 40 as a variable `gain amplifier whose voltage gain is inversely proportional to its input voltage. The variation in voltage gain is accomplished by furnishing grid 39 with no initial bias. It will be seenthat each Apositive half cycle of alternating currentvolt- A age developed across resistor 36 causes electrons to be attracted to grid 39 inV proportion to the amplitude of the positive half of the alternating current cycle. These electrons charge capacitors 31,v 59, 60 and the capacitors in filter networks '64 and 66, as well as the distributed capacity between conductor 53 and shield 54,- to a Voltage ...which-,is a negative direct current voltage with @respect to. common ground.A This negative ,volt- :Z5

age is proportional to the alternating current voltage generated across load resistor 36 and, accordingly, is proportional to the alternating current voltage generated across solenoid 28. The negative voltage thus produced furnishes the bias for tube 49 and, since the tube is a variable -mu tube, its gain is inversely proportional to the bias voltage and hence to the alternating current voltage presented to grid 39. The rectified grid voltage leaks off through resistors 5 Ia and 5|b thereby giving the control system a time constant which is proportional to the product of the resistance of resistors 5|a and 5|b and the combined capacitance of the capacitors 31, 59, 60, and the capacitors in filters 64 and 66, as well as vthe distributed capacity of cable 52.

While I have described a preferred embodiment of my invention in conjunction with Fig. 1 employing an alternating current power source 55, it is to be understood that under some conditions I may employ a direct current power In this embodiment elongated magnetostrictivemetal tube l0 is af'xed midway between Vthe ends thereof to a ring wedge 1|. Ring wedge 1| is securely seated in Ia conical seat member 12 which, in turn, is secured to an outer cylindrical housing 13. This method of mounting element I I) within housing 13 so that the magnetostrictive element is free to vibrate is specifically disclosed and claimed in copending application Ser. No. 776,549, filed September 27, 1947, for C. H. Kean, now Patent No. 2,497,172.

Cylindrical housing 13 is provided with endmembers 14 and 15. End member 14 is provided with a small passage 16 adapted to be closed by a valve or other suitable means such as a cap screw 11..Simi1arly, end member 15 is provided with a small passage 18 adaptedl to be closed by suitable means such as cap screw 19. Passages 16 and 18 are provided so that the interior of the device included between the walls of housing 13 and end members 14 and 15 may be completely iilled with a liquid such as water adapted to transmit vibrations from element I0 to the walls of housing 13.

Midway between the ends and concentrically within tubular element I9 a high conductivity, non-magnetic metal plate member is spacedly affixed to the walls of element I0 by means of a plurality of pin members 80a. Spool-like members 8| and 82 upon which are wound, respectively, solenoids |2 and 28 are securely fastened co-axially within but not touching the walls of element I0 by means of a non-magnetic metal bolt 83. Bolt 83 also secures permanent magnets and at the outer ends of the solenoid aprem-:u

endof solenoid 28 may be connected to the metal y plate 80 which may serve as a common ground return through the metal housing to the-elements -included in housing 1Q.

-f When logging subsurface formations with the device illustrated in Figs. 1 and 2, the magnetostrictive element is operated as a transducer capable of converting electric energy to acoustic energy and also capable of converting acoustical energy reected from the Walls of a borehole to electrical energy. The transducer is lowered into the borehole and moved along the walls thereof. When electrical energy from power source 55 is introduced into cable 52, an alternating voltage isy produced in solenoid I2 and causes mag- .netostrictive element I6 to vibrate at its resonant `frequency producing acoustic waves which 'are `cylindrically symmetric in nature and are radiated to the Walls of the borehole. If the Walls of the borehole adjacent the transducer are rel- 'atively hard, a major portion of the acoustic :energy is reflected back and tends to damp the vibration of element I8. On the other hand, if :the vwalls of the borehole adjacent the transducer are relatively soft, -a portion of the acoustic energy is absorbed and is not reiiected back to the transducer. Accordingly, the magnitude of vibra tion of element IB will vary with the relative vhardness ofthe walls f the borehole. By providing the electron tube network associated with electron discharge tube 40, the voltage input to solenoid I2 is maintained constant and the vary,- ing voltage produced in solenoid 28 as a result of iiuctuating vibration of element I0, caused by different degrees of damping thereof, produces aA fluctuating direct current voltage across resistors 'Sla and SIb. This voltage is a function of the acoustic energy reflected into the transducer byy the subsurface formations and is, thereforerprofy portional to a function of the acoustic impedance' of formations adjacent the transducer.

Having fully described and illustrated the-present invention, what I wish to secure by Letters l Patent is:

1. In an oscillator system including iirst andv second electron discharge tube networks, `each having at least a signal grid input circuit and an 1 anode output circuit, and including acoupled pair# of solenoids with the rst of said solenoids ele trically connected as a load impedance in t` anode output circuit of said first tube netwo and a second of said solenoids electrically corre :nected in the signal grid input circuit of said second tube network whereby a portion of the `*output energy of said system is coupled into the input circuit of said second tube network suffi?- `cient to maintain oscillation in the system, the

improvement which comprises an electron tube control network having a Voltage gain inversely substantially constant under varying load conditions v2. In a magnetostrictiontransducer system including first and second electron discharge tube ingfarmagnetostrictive.metal element and a pair of solenoids electrofmagnetically coupled by said element withra'rst of said solenoids .electrically connected .as-saidload impedance in the anode output circuit of a rst tube network and .the second of said solenoids electrically connected in the signal grid input circuit of said second'tube network whereby a portieri Vof the outputenergy of said system'is .coupled into Vthe input circuit-of saidsecond tube network sufficient toimaintain oscillationin thesystem, v.the improvement which comprises an electron -tube control network having aV voltage gain'inversely proportional to the inputvoltage thereof interposed between thelinput circuit of Ysaid first tube network-and .the output circuit of said second tubenetwork Vwhere-'- by -the valternating current voltage across Said load impedance is maintained substantially'constant under varying load conditions upon said transducer.

3. Anapparatus for logging subsurface formations penetrated by a boreholecomprising amagnetostriction transducer adapted to be vmoved through vthe borehole, an electron dischargeA tube oscillator network `electrically connected to the transducer for continuously generating an-alternating current 'voltage capable of exciting said transducer and causing it vto produce acoustic energy, an electron tube control network-having ya voltagegain inversely proportional to the input voltage thereto interposed within said oscillator network for maintaining said alternating cur rent voltage substantially constant, and means electrically connected to lthe input circuit of said control network for .continuously recording-a -vo1t age which is proportional to a function 0f the acousticimpedance of successi-ve formations along the borehole.

4. A magnetostrictiontransducersystem which includes, in combination, .a magnetostrictive metal element electro-magnetically coupled to each of a Vpair of solenoids, a first electron dis# charge tube network havingA signal grid inputand anode output circuits andhaving a rst of said solenoids connected as a load impedance in the network.

A5. A magnetostriction transducer system in .accordance with claim 4 and including an electron discharge tube voltmeter network Aelectri cally connected to thesignalgrid circuitofsaid third electron discharge tube network.

6. Amagnetostriction transducer system'adapt- `ved for usein thelogging of subsurface formations penetrated by a borehole which includes,

.in combination, la magnetostrictive element, l'a

'iirst electron discharge tube network, a rst solenoid electro-'magnetically coupled to the *mag- .netostrictive element' and' electrically connected asa load impedance in the anode circuit of said "first network', a second electron discharge` 'tube network, a second' solenoid electro-magnetically coupled tothe magnetostrictive element and electrically connected as an impedance-in 'thesignal grid circuit of said second network, a third electron discharge tube network having a voltage gain inversely proportional to the input voltage thereof interposed and electrically coupled between the anode circuit of said second network and the signal grid circuit of said first network, a rectier and lter network arranged to supply suitable direct current power to said rst, second, and third tube networks, a constant frequency alternating current power source, a cable including conductors connecting said power source to said rectifier and lter network arranged to suspend and move said rst, second, third and lter networks through the borehole, a first filter network, capable of passing direct current voltages while attenuating alternating current voltages having a frequency substantially that of said power source electrically connected adjacent said rectiner between the signal grid circuit of said third electron discharge tube network and Ythe conductors in said cable, an electron discharge tube voltmeter network, and a second lter network of similar capability to said first lter network connected adjacent said power source between the input of said voltmeter and the conductors in said cable.

7. An oscillator system comprising first and second electron tube networks each having an input circuit and an output circuit, means connected in the output circuit of said rst tube network for deriving output energy therefrom, means for coupling a portion of said output energy into the input circuit of said second tube network to sustain oscillation in the system, and an electron tube control network having a -voltage gain inversely proportional to the input voltage thereto interposed between the output circuit of said second tube network and the input circuit of said rst tube network whereby the output energy of said system is maintained substantially constant.

FRANK N. TULLOS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,088,324 John July 27, 1937 2,200,476 Mounce May 14, 1940 2,244,484 Beers June 3, 1941 2,251,817 Athey Aug. 5, 1941 2,444,349 Harrison June 29, 1948 2,451,021 Detuno Oct. 12, 1948 2,466,904 Lundstrom Apr. 12, 1949 

